Ricardo G. Hechanova
Manila, Philippines
1. INTRODUCTION
2. FORMATION OF ACID SULPHATE SOILS
3. FIELD IDENTIFICATION
4. PROBLEMS OF POND CONSTRUCTION AND PLANNING
5. REMEDIES FOR PONDS BUILT ON ACID SULPHATE SOILS
6. MANAGEMENT MEASURES
7. SUMMARY AND CONCLUSIONS
8. REFERENCES
With the increased demands for freshwater and brackishwater aquaculture, areas with potential acid sulphate conditions are increasingly being placed under cultivation. Aqua-culture is probably the most suitable use for areas with actual acidity.
The distribution of acid sulphate soils is world-wide with large areas found in the tropical deltaic and coastal zones. Tang (1979) estimated that at least 60 percent of the fishponds in the Philippines are affected by acid sulphate conditions. The low production of milkfish ponds in the Philippines for instance, could be attributable at least in part to the occurrence of acid sulphate soils.
Of the estimated 500 million ha of fine-textured soils in coastal areas, 11.4 million ha are highly pyritic, which will acidify upon aeration. The extent of actual and potential acid sulphate soils in West Africa may be about 3.7 million ha (FAO/Unesco, 1974) and smaller areas are also found in the coastal zones of the Netherlands, Sweden and Finland (Bloomfield and Coulter, 1973) .
About 6 million ha are found in Southeast and East Asia, with Indonesia having 2 million (Driessen and Suproptohardjo, 1974) and Vietnam having 1 million (Tram and Lieu, 1975), the largest share.
Improved technology in reclamation of these soils has been developed at the Brackish-water Aquaculture Centre (BAC) of the University of the Philippines in Leganes, Iloilo in the Philippines. Improved methods of construction and maintenance have been developed in Southeast Asia and elsewhere where acid sulphate soils have been reported.
The use of the prefix 'cat' in the cat-clay soil name comes from the Dutch vernacular 'kattekleigronden'. 'Cat-clay soils', the English derived from this, was connotative of harmful, mysterious influences or qualities. Traditionally, the expression 'acid sulphate soils' is used to denote soils with cat clay phenomena, those which have a very low pH and yellow jarositic mottles after drainage and aeration of the originally waterlogged parent material.
A potential acid sulphate soil or material is a soil or a reduced parent material which is expected by the person identifying it to become acid sulphate soil upon drainage and oxidation.
The genesis of acid sulphate soils is mainly in the formation of pyrite. Pyrite formation involves the bacterial reduction of sulphate to sulphide, partial oxidation of the sulphide to elemental sulphur, and the interaction between ferrous and feric iron with the sulphide and elemental sulphur. The basic factors therefore required for the formation of acid sulphate soils are a sufficient supply of sulphate, iron, high organic matter, presence of the sulphate reducing bacteria Desulfovibric desulfuricans and Desulfomacultom and an aerobic environment alternated with limited aeration.
Fine-grained iron oxide is found in sufficient quantities in the clayey sediments of tidal swamps, though iron could be limited in sandy and peaty soils. Dense mangrove vegetation supplies abundant organic matter. Fine-grained iron oxide and metabolizable organic matter are the essential ingredients required for pyrite formation (Singh, 1980).
In the zone between mean high water and mean low water, pyrite formation is most favourable due to aeration during periods of tide changes. Less pyrite is accumulated in the zone below low tide level. In the better drained zone above the high water level which is aerobic most of the time, there is less occurrence of pyrite.
Potential acidity develops gradually due to the removal from the system by tidal action of a part of the alkalinity in the form of bicarbonates (HCO3) formed during sulphate reduction.
The reaction of sulphur reducing bacteria involved in pyrite formation can be described as follows:
|
2CH2O + SO4 |
- |
H2S + 2HCO3 |
|
and with HCO3 removed by tidal flushing | ||
|
Fe(OH)2 + H2S |
- |
FeS + 2H2O |
|
and, | ||
|
FeS + S |
- |
FeS2 (pyrite) |
The final pH of the soil after drainage and drying depends on the amount of pyrite oxidized and the acid neutralizing component in the soil, as silicates, carbonates and exchangeable bases. In the humid tropics carbonates are practically absent in coastal sediments. Volcanic areas provide sediments rich in weatherable silicate minerals and the presence of such minerals provides a high amount of acid neutralizing agent. This prevents development of potential acidity in the coastal tidal flats.
There is no single commonly accepted method for the identification and for the prediction of acid sulphate soils. Actual acid sulphate soils are not common; far more abundant are potential acid sulphate soils for which recognition in undrained or waterlogged areas is generally difficult.
Evaluation of a technique for determining potential acidity was conducted in BAC fishponds in 1972 (IFP Technical Report 10). It was found that incubation of the moist soil samples in thin plastic bags for a period of 30 days appears to be an effective technique for the identification of potential acid sulphate soils.
The rate of oxidation is regulated by the iron oxidizing bacteria. If a soil is dried rapidly, the bacteria is inactivated, oxidation is slow, and it may take many months for the soil to reach its maximum acidity. If the soil is kept in a moist aerobic condition, the iron oxide bacteria thrive and the development of soil acidity is attained within a few weeks. The plastic bags allow pyrite oxidation to proceed while maintaining the soil moisture. The pH of the samples with potential acidity dropped to 4.0 or less in this time period. Yellow, jarositic mottles appeared on the sample. This technique of soil survey has been accepted due to ease of use.
The detection of actual acid sulphate soils presents no difficulty as these soils are characterized by pale yellow, jarositic mottles.
Black stained and odorous mud due to ferrous and hydrogen sulphides that turn brown upon exposure is also an indication of soil that may contain pyrite.
The occurrence of mounds of the mud lobster, Thalassina anomala in brackishwater tidal swamps indicate the occurrence of acid sulphate soils.
Acid sulphate in pond soil can be recognized by the very low pH values (below 4) measured in the pond water when it is flooded for the first time after a drying period, by the reddish iron oxides that form on the pond bottom shortly after flooding, and by the poor growth or absence of algae(Brinkman and Singh, 1982).
Association of Rhizophora, Nypa fruticans and Meleuca stands found in tidal brackish-water swamps is usually a strong indication of potential acid sulphate soils, whereas those of Avicenia are less acidic .
Surface efflorescence of water soluble aluminium sulphate, formed under strongly evaporative conditions when pyrite oxidizes at shallow depth, are commonly found in ponds which are newly constructed on potential acid sulphate soils.
Fish kills and the bitter taste of river water that drains sulphidic materials are indicative of a potential acid sulphate area.
Acid sulphate in the dikes can be recognized by the poor and spotty growth of vegetation on them several years after construction.
The sharp sour-bitter taste (like alum) of pale yellow coloured salts generally found near the base of the dikes is also a clear indication of acid sulphate salts.
A potential acid sulphate soil may have a pH near normal, but when oxidized by drying or with 30 percent hydrogen peroxide, the pH drops by about two or three units, generally below 4 (Singh, 1980).
The red lead pole test can be conducted during a field survey. Stakes coated with red lead paint are driven into the soil hydrogen sulphide generated in sulphate reduction turns the red lead marking to black within one week.
These field characteristics are often not well-defined indications. A number of complementary tests are required to determine the physical and chemical properties so as to be able to assess the degree of acidity quantitatively and to design improvement schemes and measures.
Acid sulphate conditions are not permanent although they may cause tremendous problems while they exist. The problems faced by farmers in fishponds built on acid sulphate soil include low fish yields (Rabanal and Tang, 1974), slow growth of fish, slow rate of natural fish food production, fish kills due to acidity, erosion of the pond dikes, and soft shell prawns (IFP, 1974; Potter, 1976; Camacho, 1977; Singh, 1980, 1982, and Cook, 1978).
In acidic ponds, the growth of algae is discouraged by the low pH of the water, by the dark brown of the water, and the high aluminium and low phosphate concentrations.
Poor response to phosphorous fertilizer is another indication in fishponds with acid sulphate soils.
Active iron and aluminium in acid sulphate soils are capable of precipitating phosphate as compounds at low pH and oxidizing conditions. Phosphate already bound with aluminium generally does not become available (Singh, 1982).
When fishponds are constructed, blocks of silty clay are dug and built into earth dikes. On exposure, the surface of these clay blocks becomes grey-coloured due to the oxidation of the ferric sulphide, FeS2. Pond systems which have been excavated from this typical clay present an uneconomical development in the utilization of the land for aqua-culture.
Where dikes are constructed from acid soil excavated from the pond bottom area, it is best to place the 'bad' soil at the dike bottom and make the outer dike shell of pond top soil. The poor subsoil can be used for the dike core and the outer surface can be covered with the top soil, as generally soil with low potential acidity overlies soil with high potential acidity. It was found during research studies at the BAG that the top soil (0-30 cm in depth) was less acidic than the subsoil (30-100 cm in depth) upon oxidation.
Another method of fishpond construction is to build the dikes of soil taken from alternate borrow trenches paralleling the dike. The remaining alternate trenches are levelled to the desired pond bottom elevation. This method is a sort of compromise with the dike built of half the bad and half the good soil.
A ratio of dike soil mass to pond soil mass to 0.15 m depth has been determined from recent experimental studies and research at the Brackishwater Aquaculture Centre (BAG) in the Philippines. Reduction of the amount of rainwater runoff from the dikes to the ponds is better accomplished when the size of the pond is increased with a relative decrease in dike size. A minimum ratio, for soil improvement in the least period of time, is when the mass of soil in the pond, to a depth of 0.15 m, is equal to the mass of dike soil.
If the pond bottom is low and the earth excavated is put into large dikes the effective pond area is reduced, with consequent increase in the acid runoff into the ponds. A better approach would be to build the ponds with higher bottoms and have the tidal water supplemented by pumping water into the ponds. Pumping increases operating costs, but this may be offset by reduced construction cost and by increases in production.
Experiments in unreclaimed ponds of the BAC were conducted by Poernomo in 1982 using a rapid reclamation scheme. Earlier experiments were also conducted by Camacho in 1977 at BAC and at different locations in Panay Island by Singh in 1980. The basic concept in all these studies was to remove the source of acidity by oxidizing the pyrite from the pond bottom and flushing it out. At the same time the acid materials and other toxic elements from the big dikes were also leached and removed. The details of the workplan adopted by Poernomo (1982) from Brinkman and Singh (1982) are quoted below:
'The procedure involves a precisely planned sequence of filling, draining and drying the ponds, cultivation by tooth harrow and finally broadcasting a small amount of lime in the pond soil. In the same period, the top of the surrounding dikes should be made into a series of long narrow paddies by small levees along their edges, and seawater pumped or carried into them.
A pH meter or a roll or strips of pH indicator paper should be available. A tooth harrow and draft animal are needed for the cultivation of the pond bottom. A small diesel-powered pump mounted in a small boat or on a raft of bamboo or oil drums makes it possible to rapidly inundate the tops of the dikes. About 1 ton of powdered agricultural lime per ha is also required.
The workplan can be completed in about 3 months. All the work should be done in the dry season. Treatment of the pond bottom and of the dikes should proceed at the same time. During the first heavy rains after the pond is again in operation, some further work should be done, as described below.
Treatment of the pond bottom
In the early part of the dry season, the pond has to be prepared for removal of the acid. This is done by drying the pond thoroughly. Small drains should be dug to let all remaining patches of standing water run dry. The pond bottom should be tilled after one week by tooth harrow in two directions. It should be harrowed after thorough drying (cracks to appear in the soil to about 10 cm depth) so that the surface layer is broken into small pieces. If there is no rain, the total drying period will probably take 2 to 3 weeks.The acid in the dry layer is ready to be removed. Brackishwater or salt water is brought in to fill the pond. Measure the pH of the water immediately after filling and every few hours thereafter. The pH is expected to drop rapidly from that of seawater (7 to 9) to a value lower than 4, often 3.
At the first opportunity after the pH has become constant, drain the pond and make sure that this water goes to the sea and not to any other pond. This treatment removes part of the acid.
Refill the pond and again check the pH. Again drain the water as soon as possible after it has a constant pH. Repeat the refilling and the draining process as long as constant pH is about 4 or below. This may take less than a week (4-6 refills) to about 2 weeks. When the water remains at a higher pH, drain it and thoroughly dry the pond bottom again as described above.
After thorough drying, cultivate the pond bottom and again refill as described above. This time, the pH probably will not drop as low as in the first series of filling and drying. When the pH remains above 5 after 1 to 3 drying cycles, drain and broadcast 500 kg agricultural lime per ha (not calcium oxide or calcium hydroxide) well-distributed over the pond bottom. Do not incorporate the lime into the soil. The pond is then ready to start normal operations if the dikes have also been treated.
Treatment of the dikes
At the same time as the acids are removed out of the pond bottom, the acids in and on the dikes should also be removed. Because the dikes are normally dry, the acid can be washed out without first drying as is needed for the pond bottom. Small levees, similar to the levees between wetland rice fields, should be constructed on the top of the dikes along both sides and the surface between them should be levelled. At the same time, any holes in the top surface should be filled.To avoid excessive amounts of earth removal, this bunding and levelling can be done separately for each section of dike depending on its elevation. The work should be completed by the time the pond bottom has dried out and is ready for the first filling with water.
At that time, seawater or brackishwater should be pumped or brought into the levelled paddies on top of the dikes, enough to keep them flooded to more than 10 cm depth. At first it will be necessary to check the whole top surface and the length of the small levees for leaks. Acid water will soon seep out toward the pond or the canal. Pumping of seawater from the intake canal should be continued as necessary to keep all the tops of the dikes flooded. -When the pond bottom is ready to be dried thoroughly again, stop pumping and allow the top of the dikes to dry out. If there is still some water standing after two days, drain this, to the canal if possible, otherwise through the pond. When the pond bottom has thoroughly dried and has been cultivated, the top of the dike should be flooded again during the next series of filling and draining the pond. When the pH of the water in the pond remains 5, stop flooding the top of the dike and remove the standing water.
On dikes between two ponds, remove both levees and bring this soil toward the centre. Make the top of the dikes smooth and slightly sloping from the centre to both sides. Do not leave loose soil lying about. On dikes along a canal remove only the levee along the side of the canal and bring the soil toward the other levee. Make the top of these dikes smooth and slightly sloping toward the canal.
Then broadcast 1 kg of agricultural lime per 10 m2 on the slope of the dike along each pond and 1 kg per 20 m2 on the top of the dikes between the ponds.
The same problem of acidic rainwater runoff from the dikes to the ponds was encountered at the Brackishwater Aquaculture Centre in Gelang Patah, Johore Bahru in peninsular Malaysia, causing fish kills and retarded growth of the fish. The acidic runoff was found to be potentially due to acid sulphate condition Of the soil material of the dikes which are massive and with flat side slopes.
The usual amount of chicken manure of 2 t/ha is distributed over the pond bottom. To increase the rate of phosphate fixation, rice hulls, ash of rice hulls or filterpress mud from the sugar mills can be spread over the pond bottom before spreading the chicken manure. A few days later, initial amounts of nitrogen fertilizer are broadcast in the pond water. In contrast to the usual practice in non-acid fishponds of broadcasting the recommended phosphate amounts once every 2 or 3 weeks, the phosphate should be divided into portions and broadcast every 2 days, or weekly portions should be placed in jute bags on floating platforms at two platforms per hectare, to dissolve slowly. By these methods the Phosphate concentration in the pond water can be kept high enough for good growth of algae, without excessive amounts of fixation on the material of the pond bottom.
Experimental and research work was conducted at BAC by D.J. Hechanova from January to August 1981 on the effects of various organic materials on the changes in the chemical properties of soil and water systems of submerged acid sulphate soils. Seven treatments were used including the application of burnt rice hulls, partly decomposed rice hulls, chicken manure at 2 t/ha, and Fertilex, an organic commercial fertilizer, all in combination with mono ammonium phosphate (16-20-0), and mono ammonium phosphate alone. Marfon, a fertilizer/soil conditioner produced from fermented rice hulls was also used at 5 t/ha.
The findings showed that the Marfon and chicken manure applications showed the lowest concentration of sulphates resulting in the soil and the supernatant The decrease was due to the reduction of sulphate or sulphides under anaerobic conditions and also due to the high organic matter content of these materials.
Low rates of liming give more favourable results than large-scale liming. Regular application at 500 kg/ha of powdered lime on the pond bottom before the first flooding of the pond speeds up the reduction and lowers peak concentrations of toxins. Toxins may be released into the pond water following the application of organic material. The lime is not incorporated into the soil.
Phosphate should be made available by a slow release or in frequent and small doses. Slow release is attained by the construction of a fertilizer platform inside the pond at such elevation so that the water surface is above the platform to wet half of the fertilizer mass, usually contained in a sack. The fertilizer on the platform is released in small doses as it is partly wetted.
Prior to pond development, a detailed soil survey should be conducted and when the area is identified to be potential acid sulphate soil, appropriate designs and construction methods should be adopted.
If no growth of algae is observed within a week, the water should be inoculated with algae collected from normal fishponds.
The pond water pH should be monitored regularly and if it becomes acidic and below the tolerable limit, a small quantity of agricultural lime should be broadcast in the water, but when the pond water becomes turbid it should be replaced by new brackishwater before fertilization. The new pond water is left to stay for a week after fertilizer and lime application.
Fish culture is started in the first year after reclamation, with stocking of older and heavier fingerlings than is usual for non-acidic ponds.
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